Disorders of pregnancy, including preeclampsia and intrauterine growth restriction (IUGR), are associated with placental hypoxia and elevated expression of HIF-1α [20, 21]. Hypoxia is also an important physiologic inducer of tumor metastasis and HIF-1α levels correlate with hypoxia in solid tumors. Since HIF-1α is central to proper placental development and cancer, early detection of aberrant HIF-1α regulatory mechanisms could impact on the diagnosis of pregnancy-related disorders and cancer.
The highly conserved hypoxia-inducible family of transcription factors is a major player in the physiological response to chronic and acute hypoxia [4]. The family consists of heterodimers comprised of one of three alpha subunits (HIF-1α, HIF-2α and HIF-3α) and a beta subunit (HIF-1β). Under hypoxic conditions the alpha subunits are stable, allowing it to accumulate in the nucleus, where upon binding to HIF-1β it recognises HIF-responsive elements (HRE) within the promoter regions of hypoxia-responsive target genes. Under normoxic conditions, the alpha subunits are rapidly degraded by means of ubiquitination and proteasomal degradation [5,6,7,8]. The ubiquitination process requires the product of the von Hippel-Lindau tumor suppressor gene (VHL), which functions as a substrate recognition component of an E3 ubiquitin ligase complex [5,6,7,8]. The most extensively studied isoform of the α-subunits is HIF-1α. Oxygen-dependent prolyl hydroxylases control the abundance of HIF-1α by hydroxylating two specific proline residues (402 and 564), an event which is required for VHL binding and subsequent HIF-1α degradation [9,10]. The prolyl hydroxylase-domain containing proteins 1, 2 and 3 (PHD1, PHD2 and PHD3) function as oxygen sensors as they require O2 as co-substrate to catalyze the prolyl-hydroxylation reaction, indicating that oxygen levels directly influence their enzymatic activity [11,12,13]. Moreover, in vitro experiments have shown that PHDs mRNA levels are up-regulated in conditions of low oxygen [14], further highlighting their role as O2 sensors. In contrast to HIF-1α, the stability of PHD1 and PHD3 decreases under hypoxic conditions [15]. Recent studies have shown that under hypoxic conditions, PHD1 and 3 are degraded by specific E3-ubiquitin-ligases, termed SIAHs [Seven In Absentia Homologues][15,16]. There are two known human SIAH genes, SIAH-1 (that encodes for two different isoforms: SIAH-1a and SIAH-1b) and SIAH-2. Like PHDs, hypoxia stimulates their transcription and induces the accumulation of these ring finger proteins through an HIF-independent manner [15]. Under hypoxic conditions, SIAHs promote degradation of PHD1 and PHD3 [15,16], leading to an increased accumulation of HIF-1α, whereas under normoxic conditions PHDs are stable and hydroxylate HIF-1α to target it for degradation [9,10].
Another oxygen-dependent mechanism of HIF-1α regulation involves the Factor Inhibiting HIF (FIH), an asparginyl hydroxylase that targets the Asn803 residue in the C-TAD domain for hydroxylation. This post-translational modification prevents C-TAD binding to the transcriptional activator p300/CBP, thereby repressing HIF-1α transcriptional activity [17,18]. Like PHDs, FIH has also been characterized as an oxygen sensor since its enzymatic activity is directly regulated by O2 concentration [19].